Distribution and Storage Characteristics of Soil Organic Carbon in Tidal Wetland of Dandou Sea, Guangxi
1. Introduction
The coastal zone is the interface between land and sea, possessing unique marine and terrestrial characteristics, and constitutes a dynamic and complex natural system [1,2]. Coastal wetlands, especially, hold a crucial position in addressing global climate change [3]. According to the definition of the Ramsar Convention, coastal wetlands extend from 6 m below sea level (often extending to the outer edge of large seaweed growth areas) to areas above the high tide line connected to freshwater or brackish lakes and marshes in the hinterland, as well as river sections upstream of seawater ingress [4]. Based on data from the United Nations Environment Programme (UNEP), the global area of coastal wetlands is approximately 34 million hectares, accounting for only about 3% to 5% of the world’s land area, yet around 12% of the world’s soil organic carbon (SOC) is stored in wetlands [5,6,7,8]. Therefore, wetlands play a crucial role in regulating the global carbon balance of terrestrial ecosystems [9,10,11].
There are many factors influencing SOC in wetlands, such as soil physicochemical properties [12,13], hydrological conditions [14], microbial types [15], and biophysical factors like climate [16,17]. However, SOC in wetlands is also influenced by different wetland types. For example, the study by Ji et al. [18] indicated that different land use types in wetlands with varying vegetation cover significantly affect the concentration, distribution, and chemical structure of organic carbon in different density and particle size fractions. Vaughn et al. [19] investigated the organic carbon content in mangroves, salt marshes, and mixed areas of mangroves and salt marshes in northern Florida. The results showed that the organic carbon content in the mixed vegetation of mangroves and salt marshes is higher than in the adjacent areas of mangroves and salt marshes. Different geomorphological conditions also affect the content of SOC in wetlands. By comparing the non-degraded and degraded marine and estuarine mangroves within the Indonesian archipelago, Weiss et al. [20] concluded that the SOC content in natural marine mangroves is much higher than that in estuarine mangroves. De Jong Cleyndert et al. [21] explored the effect of ocean distance on the SOC content of mangroves in Lindi, Tanzania. The results showed that SOC was significantly negatively correlated with ocean distance, and SOC content decreased with the increase of distance from the sea. However, this is different from the results of the study in Micronesia, where the SOC is greater with the distance from the sea, which is mainly due to the deeper soil depth [22]. Donato et al. [23] found that in the estuaries and marine mangroves of the Indo-Pacific region, SOC did not change with the increase of distance from the sea, which may be due to the fact that all the samples were within 200 m from the edge of the ocean. It can be seen that soil erosion and soil depth are also important factors affecting SOC. In addition to common soil physical and chemical factors such as soil bulk density (BD), pH, and electrical conductivity (EC), salinity may also be a factor affecting wetland SOC. By using ultrapure water and artificial seawater to sequentially extract the soluble organic matter (WEOM) in mangrove soil, Kida et al. [24] proposed that high salinity may be one of the mechanisms leading to SOC accumulation in mangrove soil. However, Weiss et al. [20] pointed out that salinity did not affect SOC content. Whether salinity is a factor affecting wetland SOC remains to be further studied.
Additionally, changes in wetland types can lead to substantial variations in SOC within the same region. Sun et al. [25] investigated the changes in SOC under the invasion of Spartina alterniflora (S. alterniflora) in mangroves, revealing that the invasion increases the rate of SOC decomposition, thereby hindering its accumulation. Similarly, Wang et al. [26] examined SOC content changes during the afforestation of mangroves in tidal flats, demonstrating a significant increase in SOC content after the tidal flats evolved into mangroves. Ebrahem et al. [27] evaluated the impact of land use change on SOC caused by the conversion of mangroves into shrimp ponds. The results showed that the conversion of mangroves into shrimp ponds would lead to the loss of SOC reserves, which proved that human factors would lead to the reduction of SOC reserves, and human activities were also one of the factors affecting the SOC content of wetlands.
The current study mainly focuses on the SOC content of a single wetland category and its influencing factors, or the change of SOC when a certain wetland evolves into another wetland. For example, under different geomorphological conditions, the differences in organic carbon between mangrove ecosystems, as well as the effects of salinity, pH, soil texture [28] and other factors on SOC and soil organic carbon stock (SOCS). In addition, the impact of invasive species on SOC and SOCS in tidal wetlands is also a research focus. For example, the effects of S. alterniflora invasion of mangroves on SOC and SOCS. However, there are relatively few studies on the SOC content and SOCS characteristics of the whole tidal wetland, as well as the effects of various soil physical and chemical properties and related carbon components on the SOC and SOCS of the whole tidal wetland and the sub-categories of tidal wetland.
Therefore, in order to further explore the distribution characteristics of SOC in different wetland soils and their influencing factors, this study has utilized the Google Earth Engine (GEE) platform, combined with Sentinel–2 imagery and random forest algorithm, to generate higher-resolution classification data of Guangxi tidal wetlands. Based on this data, a spatial instead of temporal approach has been adopted to investigate the variations in SOC and SOCS content across different soil layers (0–20 cm, 20–40 cm, 40–60 cm) in the Dandou Sea and Tieshan Gulf areas of Guangxi. Additionally, the correlation between SOC, SOCS, soil physicochemical properties, and other carbon components have been analyzed. This study not only fully understands the distribution characteristics of SOC in Guangxi tidal wetlands, but also finds the influence of different soil depth on SOC content. It has important theoretical value for further understanding the carbon storage of tidal wetlands and providing scientific basis and technical support for wetland protection and management.
5. Conclusions
In previous studies, many global wetland data and regional wetland data have been published. For example, Zhang et al. [49] generated the world ‘s first 30 m wetland product in 2020; the East Asian tidal wetland data with 10 m resolution in 2020 generated by Zhang et al. [51], but there is still a lack of continuous 10 m tidal wetland dataset for the coastal zone of Guangxi. Therefore, by integrating the existing tidal wetland data, based on the Sentinel-2 L2 A time series images, combined with the random forest algorithm, this study generated a 10 m tidal wetland dataset in Guangxi from 2019 to 2023, including mangroves, salt marshes and tidal flats. Subsequently, based on the classification results of tidal wetlands in 2023, the method of time instead of space was used to explore the changes of soil organic carbon and soil organic carbon storage under different habitat categories of tidal wetlands.
The results showed that the overall accuracy of tidal wetland identification in Guangxi from 2019 to 2023 was higher than 96%, and the Kappa coefficient was higher than 0.95, indicating that the identification results had high accuracy. Compared with other tidal wetland data, the phenomena of misrecognition and missed recognition are reduced, which proves that the research method is suitable for the identification of tidal wetlands in complex landform areas. Quantitative statistical analysis shows that in Guangxi’s tidal wetlands, tidal flats account for about 80% of the total area, mangroves account for about 15%, and salt marshes account for less than 10% of the total area. Additionally, experimental results indicated that in the surface layer (0–20 cm) of soil, soil organic carbon and soil organic carbon storage were ranked in the following order across habitat types: mangroves > salt marshes > tidal flats. Whereas at depths of 20–40 cm and 40–60 cm, soil organic carbon and soil organic carbon storage were ranked as follows: salt marshes > mangroves > tidal flats. The study results also suggested that certain soil physicochemical factors, such as bulk density, moisture content, and pH, were among the driving factors influencing soil organic carbon and soil organic carbon storage in tidal wetlands.
However, there are still many uncertainties and limitations in the proposed wetland identification methods and identification results. In the process of generating stable sample points, the collected tidal wetland datasets are current up to 2020, and after that, the wetland category may change, which may lead to a certain proportion of errors in the sample points automatically generated based on the existing datasets. Therefore, it is necessary to further collect multi-source datasets to fill these gaps and improve the accuracy and comprehensiveness of wetland identification. In addition, in this study, only four carbon components of soil organic carbon, dissolved organic carbon, easily oxidized organic carbon and particulate organic carbon, were explored. Other carbon components in the soil were not analyzed and studied, such as mineral-associated organic carbon, microbial biomass carbon, etc. In the future, it is necessary to further study the soil carbon components and soil carbon pool composition of tidal wetlands in Dandou Sea area of Guangxi. This is conducive to a more comprehensive understanding of the carbon storage and cycle processes of wetland ecosystems, and provides a scientific basis for wetland protection and sustainable development.
